Documents such as booklets, packets, and the like often consist of subsets of printed sheet material that are bound together by perfect binding or other finishing techniques. The various subsets comprising each document can contain one or more units of sheet material. The subsets are often supplied from different sources such as color printers, black and white printers, and offset printers. Each subset might have been printed at a different time and place, so that the subsections must be subsequently merged to form a complete document. In order to assemble a large volume of documents, each containing multiple subsets of sheet material, multiple print streams must be merged. The merging of multiple print streams is typically done manually, and accordingly can be time consuming, create health problems due to repetitive motion, and result in an unacceptable rate of integrity defects due to human error. These and other problems can be more acute in processing jobs where each document, while containing the same types of subsets, is personalized such that one or more of the subsets includes information specific to the individual intended to receive that document.
Accordingly, the desirability of automating the process of merging multiple sheet streams is well recognized within the industry. As a general matter, the merging of sheet materials can be performed by collating machines, but conventional collators are not optimized for assembling a series of personalized documents from multiple input streams. Typical collators are capable of accumulating only single sheets. Moreover, typical collators are order-dependent, meaning that their input streams are fixed such that the accumulation or collating process cannot be modified or randomized. In addition, the scrap cost associated with conventional collators is unacceptably high due to the required use of separator sheets. Separator sheets are used to mark or identify each subset of sheet material within the stack comprising a complete document. Such separator sheets are typically discarded upon completion of the document, and in any event do not add value to the information provided by the document.
An example of a system for collating multiple incoming sheet streams is disclosed in U.S. Pat. No. 5,462,399. Like other conventional collators, the disclosed system is order-dependent. The system includes three input devices oriented at right angles to each other. Each input device feeds sheets into a centrally located collating device. The collating device is constructed from a stack of three vertically spaced trays. Each input device is limited to feeding its corresponding sheets into a specific one of these trays. Thus, after each input device has been operated, the collating device contains three separate stacks of sheets and hence does not itself truly merge the three input streams. A kicker arm, spanning the height of the entire collator, is then activated to push the stacks of all three levels into an exit device. Due to the configuration of the three-level collator, three distinct sets of sheets are maintained after being supplied from the three input devices. The disclosed system therefore cannot be randomized with respect to the relative order in which sheets enter the collator from multiple directions.